ACCUMULATED OTHER COMPREHENSIVE INCOME (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	$ 804,479
Reclassification to retained earnings due to change in federal corporate tax rate	(392)
Adjusted balance, January 1, 2018			$ (1,672)
Reclassification adjustment for gains on investment securities included in earnings, net of tax of ($8) and $0	(29)	$ 0
Current year changes, net of tax	(9,122)	(151)
Balance at end of period	868,944	758,216
Unrealized Gain (Loss) on Derivatives
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	292	176
Reclassification to retained earnings due to change in federal corporate tax rate	(53)
Adjusted balance, January 1, 2018			239
Reclassification adjustment for gains on investment securities included in earnings, net of tax of ($8) and $0	0
Current year changes, net of tax	281	43
Balance at end of period	520	219
Unrealized Gain (Loss) on Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(1,572)	(1,234)
Reclassification to retained earnings due to change in federal corporate tax rate	(339)
Adjusted balance, January 1, 2018			(1,911)
Reclassification adjustment for gains on investment securities included in earnings, net of tax of ($8) and $0	(29)
Current year changes, net of tax	(9,403)	(194)
Balance at end of period	(11,343)	(1,428)
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(1,280)	(1,058)
Reclassification to retained earnings due to change in federal corporate tax rate	(392)	0
Adjusted balance, January 1, 2018			$ (1,672)	$ (1,058)
Balance at end of period	$ (10,823)	$ (1,209)